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Kizmo, from your waveforms you look a bit out of tune. The inverter current is supposed to be in phase with the voltage so you can have ZCS. That's more important than ZVS, especially when pushing the limits of slow IGBTs.

Well, surely you should be using the inverter output current as the feedback source, not the tank current.

With the series resonant heater, the two currents are in phase because they're the primary and secondary currents of the transformer. But in the LCLR they're out of phase, because a phase shift is necessary to transfer real power through an inductor.

A simple feedback circuit will oscillate at whichever resonant frequency has the highest loop gain. So, you could include a highpass filter in the feedback circuit to lower the loop gain at 6kHz, and make it prefer the frequency you want.

If you can't easily design a filter that blocks 6kHz without unwanted phase shift at the operating frequency, then design the best filter you can and make the DC block cap bigger to lower the frequency of the unwanted resonance. Although, the phase shift at the operating frequency might even be beneficial as it would work like a "Prediktor".

I suppose I have to agree with you, since I've only ever built series resonant heaters!

I chose series resonant because:

The inverter output current is in a fixed ratio to the work coil current, so a single overcurrent detector can measure and limit both. If you put a limit on the PLL tracking range, it can also protect the tank cap against overvoltage. (Tank cap voltage is proportional to current and 1/frequency.)

The feed transformer isn't that much bigger/harder to build than the matching inductor of a LCLR. The only problem is the massive water-cooled secondary winding, but we 4hvers enjoy building things like that

The feed transformer isolates the work circuit from the inverter, which is usually not isolated from the mains.

But Richie says the LCLR is theoretically better. I can't remember why, maybe he can comment?

Almost done. One question: With transformer fed series resonant topology i have to include transformer high current side winding inductance with work coil and wiring inductance when figuring out the resonant frequency?

To avoid excessive inductance, I built my transformer right into the heat station as an extension of the capacitor's water cooling plates. That also got the secondary water cooled as a bonus. And I wound the primary inside the secondary, on the same core limb, again to minimise leakage inductance. (Since the inverter is a voltage source, it's the leakage inductance that's relevant.)

But a little extra inductance may not be a problem, it will reduce the change in operating frequency with workpiece type.

Kizmo, at your power levels I would suggest liquid cooling. You will loose IGBTs from overheating.

Yeps that is of course another option but i would like to fix the main cause of heating which is switching losses during turn off. And that is of course caused by poor power factor of this type of feedback method.

This whole transformer idea is starting to feel stupid. I cant have more than 5uH of inductance from transformer or this thing will turn into ear wrecking machine. And there is no way i can fit enough windigs on any of my smaller ferrites :(

How are you calculating the inductance? It's the leakage inductance, so to a first approximation, it's the inductance of the secondary by itself with the primary and core removed. In practice slightly less, if you wind the primary close to the secondary as I did.

That should be much less than 1uH. To keep a sense of scale, I remember that a 10 inch diameter loop has an inductance of about 600nH.

Kizmo, at your power levels I would suggest liquid cooling. You will loose IGBTs from overheating.

Yeps that is of course another option but i would like to fix the main cause of heating which is switching losses during turn off. And that is of course caused by poor power factor of this type of feedback method.

Kizmo, at your power levels I would suggest liquid cooling. You will loose IGBTs from overheating.

Yeps that is of course another option but i would like to fix the main cause of heating which is switching losses during turn off. And that is of course caused by poor power factor of this type of feedback method.

What is your frequency?

Limited by capacitor between 16 and 32kHz. This first prototype ran at 44kHz and at this frequecy calculated turn-off losses (=hard switching) are almost 800W/transistor.

Iamsmooth: I've levitated a small piece of aluminium tube, but I had to push the setup to the absolute limit. I turned the current limit up too high, and when the piece melted, it got unstable and fell out of the coil, and my IGBTs went boom.

Yes, if you drive it at the correct frequency, you get unity power factor just the same as the series resonant.

The correct frequency is not the resonant frequency of the tank circuit, but slightly above, where the tank circuit appears as a capacitive reactance of the right magnitude to cancel the inductive reactance of the matching inductor.

Ameritherm's small heaters use series resonant. I got the idea from some gut shots that were shared in the 4hv chat years ago.

As I mentioned earlier, I used series resonant, got it up to 12kw of power and did all kinds of crazy stuff. At these power levels you need to make sure you stay slightly above as Steve mentioned. If you start get to the true resonance or below the switches will go boom. I put a 60A fast-blow going to the inverter to protect my board from secondary frying if the switches shorted.

I minimized my switching loses with lower frequencies and fast turnon/turnoffs. One issue with faster transitions is the ringing, which has to get snubbed, or else you will have a worse situation. As far as the frequency: while I used lower frequencie for melting bricks of iron, I used frequencies close to 120 kHz, but was still able to get rid of the heat without compromising the mosfets.

You might want to consider using mosfets if you are using frequencies above 20-30khz.

As I mentioned earlier, I used series resonant, got it up to 12kw of power and did all kinds of crazy stuff. At these power levels you need to make sure you stay slightly above as Steve mentioned. If you start get to the true resonance or below the switches will go boom. I put a 60A fast-blow going to the inverter to protect my board from secondary frying if the switches shorted.

DRSSTCs are switched at perfect resonance at current zero crossings and they dont blow up...?

EDIT:

I guess i have to explain my idea of how im going to do this heater.

Series resonant tank circuit. 10µF 1kA celem capacitor, about 3µH work coil + stray inductance. Resonates roughly at 29kHz. Coupling transformer is just large ferrite transformer with 1:10..1:20 ratio. Feedback is taken from tank circuit current with measuring capacitor (150nF cap in parallel with celem and small ferrite CT). Inverter is igbt half bridge powered by rectified 3 phase and im not planning to use any filter capacitance on bridge, just couple µF worth of film caps. Driver is normal DRSSTC driver with interrupter.

Interrupter would be outputting pulses with 0-99% ON time. Idea behind this is to be able to have at least some sort of power control by varying on time. Drsstc has of course zero crossings synchronized over current protection which in this case would be measuring inverter output current. I guess if needed i could add another overcurrent protection circuit that measures tank current via same measuring capacitor. That would serve as tank capacitor over current/voltage protection.

Due to my limited knowledge, i dont see why this kind of contraption wouldnt work. At least it has proven itself in drsstc world. If you see potential problems, please stop me before i waste my time ;)

> But Richie says the LCLR is theoretically better. I can't remember why, maybe he can comment?

The work-coil circuit is really just an impedance matching network to convert the unfeasibly low impedance of a lump of solid metal up to a figure that is realistic to present to a solid-state inverter running directly off the mains line. I don't think it really matters how you achieve this impedance transformation as long as it is done efficiently because the currents and voltages at work in IH are large.

I prefer the LCLR network for a number of reasons:

1. It keeps the enormous induction-heating current confined to the work-coil / tank capacitor loop. This loop can be made very small and may be located remotely from the inverter.

2. It does away with the need for a high-power high-frequency transformer requiring careful design to minimise core/copper losses and water cooling to carry away the heat from whatever dissipation remains.

3. It is better to alter the impedance matching by varying the air gap in the matching inductor than to change taps on a transformer. A multi-tap transformer implies poor winding window utilisation for certain tap choices resulting in an overall bigger transformer unless series/parallel winding combinations are used.

4. Stray inductance in the wiring between the inverter and the heat station merely adds to the matching inductance, rather than adding to the work coil inductance and reducing coupling to the workpiece.

5. The LCLR network presents a light inductive load to the inverter in the case of a tank capacitor or work-coil short circuit making the system s/c tolerant to faults at the heat station.

6. If the matching inductor is located close to the inverter it reduces HF switching noise leaving the enclosure improving EMI performance.

7. In high-power systems the tank circuit can be fed from several inverters in parallel through several matching inductors. The "ballasting action" of the matching inductors provides inherent current sharing. It also limits the "shoot-between" currents resulting from imperfect syncronisation of the inverters' switching transitions. (This way solid-state systems in the MW and 100's of kHz range can be realised with a high level of redundancy.)

The downsides of the LCLR matching network are:

1. It is more complex to understand the resonance transformation than impedance transformation with a transformer.

Richie, you must have seen my schematic for the work coil. It is a coupling transformer from the switches (10-20:1) around the wire going to a series LC tank. If I were to improve the tank circuit, what would I need to do? A different topology or is this one reasonable? I was able to abuse it without failure, but I'm always interested in other and better ways of doing things.

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